1. Field of the Invention
The invention is generally related to pharmaceuticals which are useful for allosterically modifying pyruvate kinase.
2. Description of the Prior Art
Mammalian pyruvate kinase (PK) is a key regulatory glycolytic enzyme that exhibits allosteric kinetic behavior. The basic mechanism of the allosteric regulation of PK at the molecular level is still not known. There are reports that PK undergoes conformational changes and that the changes involve domain events. PK is found in all cells and tissues. It catalyzes the conversion of phospho-enolpyruvate (PEP) and adensosine diphosphate (ADP) into pyruvate and adenosine triphosphate (ATP), as shown in reaction scheme 1. EQU PEP+Mg.ADP+H.sup.+.fwdarw.(Mg.sup.2+, K.sup.+).fwdarw.Mg.ATP+Pyruvate (1)
The reaction proceeds in two steps. First, the 2-phosphate is removed from PEP to yield ATP and the enolate ion form of pyruvate. The second step involves the protonation and tautomerization of the ion to yield the keto form of pyruvate. The enzyme requires three cation cofactors, two divalent (Mg.sup.2+ or Mn.sup.2+) and one monovalent (K.sup.+). Positive factors of PK include fructose 1,6 diphosphate, PEP, and low pH. Negative factors of PK include ATP, high pH, and glycogenic amino acids such as alanine and phenylalanine. The products of reaction scheme 1, pyruvate and ATP, are involved in a wide variety of metabolic pathways; therefore, PK can be considered a key enzyme in the glycolytic pathway as well as many other pathways in the cellular metabolism.
In mammals, PK has four isoenzymes which are identified as M-1, M-2, L and R type. The R type PK exists exclusively in red blood cells and its biochemical properties change with cell maturation. Congenital R-PK deficiency in erythrocytes is one of the most frequent enzymopathies involving the glycolytic pathway. It is an autosomal recessive disorder, and in hemozygotes, causes nonspherocytic hemolytic anemia. Heterozygotes do not show symptoms of hemolytic anemia, but have lowered residual PK activity, with increased 2,3-diphosphoglycerate (2,3-DPG) and fairly stable ATP levels. Thus, it appears quite possible that similar results will be obtained pharmacologically from partial inhibition of erythrocyte PK. Because ATP can be synthesized in erythrocytes only by glycolysis, a decreased PK activity causes severe disturbances of the erythrocyte energy metabolism and leads to greatly diminished lifetime of the red blood cells.